To facilitate client-side reporting of RUM, CirrusSearch should embed relevant performance data in the page output.
This could be done by defining a MakeGlobalVariablesScript hook handler, or by adding a call to $out->addJsConfigVars() in CirrusSearch\Hooks::specialSearchResultsPrependHook.
(Please don't use <!-- comments -->; having to parse the data from a comment node sucks.)
As to what data should be sent: Navigation Timing data will tell us the total time it took the user's browser to retrieve and render the search result page, so what we want are figures that can tell us the portion of that time that was taken up by Cirrus. If you want to be very granular, and provide timing data for different parts of the total request processing time, that's fine too.
Please use milliseconds as the unit; fractional milliseconds are OK if you want finer resolution.